The front fixing plate is an aluminium bar with threaded inserts on either end. The ends are thinner in depth to stop the fixing plate slide between the frame bars either side of the cradle.
I'd get a bit of flat steel bar, say 3/4" wide, and drill holes in it spaced the same as the fitting holes on the skid plate, then weld nuts behind the holes.
Pass bolts, through and tighten up.
The rear is fixed with a hook shaped bit of aluminium, with a threaded insert, again, the hook hooks a bit of the frame under the engine, and you pass a bolt through the skid plate, and thread into the hook.
I'd get a bit of 2" scaffold pipe, cut a 1/2" section off, chop that in half, making a crescent of material, bend that into a slightly tighter bend toward one end.
Weld a nut to the other end, and hey presto, a hook!
May be worth trying to download / find the instructions for any skid plate, they all fit in a similar way.
